Biodiversity Net Gain Small Sites – New Guidance

As it is now a part of the Environment Act 2021, Biodiversity net gain (BNG) will become mandatory for major developments on 12 February 2024 and for small sites in April 2024. BNG refers to the process whereby a development considers the environmental impact in terms of biodiversity.

It aims to ensure that biodiversity is left in a better state than before the development was completed. This is done by avoiding biodiversity loss, retaining various habitat types and protecting any assets of strategic significance.

A draft of the secondary legislation package was released to assist stakeholders in readiness for mandatory BNG. Notably, there has been significant attention to the de-minimis exemption and the biodiversity gain hierarchy. 

Specific procedures must now be followed for both major developments and small sites in order to facilitate the project’s progression and obtain planning permission.

What is considered a small site?

Small sites include residential development sites where the number of dwellings is between 1 and 9 or if the site area is less than 0.5 hectares. Small sites also include commercial developments where floor space created is less than 1000 square metres, or the overall site area is less than 1 hectare.

Biodiversity net gain will be made mandatory for new planning applications under the Town and Country Planning Act 1990. As a result of this, local planning authorities will now refuse any applications that do not reach the governments BNG requirements. Developers will also have to guarantee that BNG is maintained for at least 30 years.

Small sites were given a phased introduction to the new BNG rules in order to let the rules settle and give local authorities more time to become accustomed to the regime.

What level of net gain must be achieved?

You must be able to demonstrate that your project will lead to a quantifiable rise in biodiversity of no less than 10%. This percentage increase will be determined by:

  1. The biodiversity within the red line boundary after development compared to the level before
  2. Biodiversity gains off the site that are registered against the site
  3. Biodiversity credits bought for the development from the Government

In certain instances, the Local Planning Authority might request a percentage increase higher than 10%.

Calculating biodiversity net gain

Metrics assign every habitat on a biodiversity gain site a ‘biodiversity unit value’ according to its relative importance for biodiversity. This enables comparison between the existing value of a site and what will be delivered through development or management and post development. This may include an increase in natural habitats through retention and enhancement and/or creation, which goes over and above the environmental habitat originally achieved on site.

The UK Government has published the most recent version of the BNG metric tool. BNG can be calculated through the DEFRA biodiversity metric 4.0, which requires a limited number of factors. The government website also provides a biodiversity metric 4.0 calculation tool, which can help to determine your biodiversity unit score. This will then translate into the standards of your local planning authority.

Implementing BNG

In order to obtain planning permission and reach permitted development status from your local planning authority, you must be able to prove that you are taking the correct measures to increase biodiversity net gain. One of the first steps in this process is to book an experienced ecologist to create and develop a biodiversity net gain plan.

Depending on the ecologist’s findings within a BNG assessment, the BNG plan will help to determine natural elements that could potentially be at risk as a result of the development project and any mitigation methods to prevent these outcomes from occurring.

If you are unsure of how mandatory biodiversity net gain may affect your planning application, you should contact your local planning authorities immediately. They can discuss your development plans in detail and whether they align with BNG.

At Collington Winter, our team has strong experience completing BNG and will provide guidance throughout the planning process.

How can Collington Winter assist?

Our team of ecologists and land managers have helped numerous clients over the years, including policy guidance for delivering BNG. 

We can assist with biodiversity net gain by providing: 

Please get in touch if you would like further information about further biodiversity net gain small sites guidance. We are happy to offer free CPD sessions on the BNG principles and how we can help your schemes achieve this.

Our Ecology Director, Olivia Collington, holds a Natural England license. If you would like to find out more about the services we provide, feel free to contact us using the details below.

Contact Us

Registered Address

23 Bark Street East, 1st Floor, Bolton, BL1 2BQ

Cambridge Office

Future Business Centre, Cambridge Campus, Kings Hedges Road, Cambridge, CB4 2HY


Head Office: 01204 939 608

Dumfries Office: 01387 378208

Get in touch
close slider
Please enable JavaScript in your browser to complete this form.